1. Field of the Invention
The present invention relates to an optical wavelength converter device, more particularly, an optical wavelength converter device which comprises a cladding and a waveguide disposed in the cladding and made of a nonlinear optical material having a refractive index which is higher than that of the cladding, for achieving phase matching between a fundamental wave and a wavelength-converted wave which travel in guided modes.
2. Description of the Prior Art
Various attempts have heretofore been made to convert the wavelength of a laser beam into a second harmonic (i.e., to shorten the wavelength of a laser beam), using a nonlinear optical material. One well known example of an optical wavelength converter device for effecting such laser wavelength conversion is a bulk-crystal-type optical wavelength converter device. This optical wavelength converter device relies upon the birefringence of a crystal in order to meet phase matching conditions. Therefore, any material which does not exhibit birefringence or exhibits only small birefringence cannot be employed, even if it has high nonlinearity.
To solve the above problem, there has been proposed a fiber-type optical wavelength converter device. An optical wavelength converter device of this type is in the form of an optical fiber comprising a core made of a nonlinear optical material and surrounded by a cladding. One example of such an optical fiber is shown in Vol. 3, No. 2, pages 28-32, of the Bulletin of the Microoptics Research Group of a gathering of the Japan Society of Applied Physics. Recently, much effort has been directed to the study of a fiber-type optical wavelength converter device since it can easily achieve phase matching between a fundamental wave and a wavelength-converted wave. There is also known an optical wavelength converter device in the form of a two-dimensional optical waveguide sandwiched between two substrates which serve as a cladding and made of a nonlinear optical material, as disclosed in U.S. Pat. No. 4,820,011, for example. Another known optical wavelength converter device comprises a three-dimensional optical waveguide embedded in a glass substrate and made of a nonlinear optical material, for emitting a second harmonic into the glass substrate. These known optical wavelength converter devices are also capable of easily achieving phase matching between a fundamental wave and a wavelength-converted wave.
U.S. Pat. No. 4,952,013 shows in detail a fiber-type optical wavelength converter device which can generate sum and differential frequencies. The generation of sum and differential frequencies with a waveguide-type optical wavelength converter device is also disclosed in detail in U.S. Pat. No. 4,952,013 It is also possible to generate a third harmonic wave using tertiary nonlinearity.
The above optical-waveguide-type optical wavelength converter devices (including those of the fiber type) are roughly classified according to phase matching process, as follows:
(1) The Cerenkov-radiation type which achieves phase matching between a radiation mode in which a wavelength-converted wave is radiated into a cladding and a guided mode in which a fundamental wave is guided; and PA1 (2) The waveguide-waveguide type which achieves phase matching between a fundamental wave and a wavelength-converted wave which travel in guided modes.
Optical waveguide converter devices of the waveguide-waveguide type (2) can expect in principle higher wavelength conversion efficiency due to greater interaction between a fundamental wave and a wavelength-converted wave than optical waveguide converter devices of the Cerenkov type (1). More specifically, if it is assumed that the length of interaction (i.e., the length of the optical waveguide) is indicated by L, the wavelength conversion efficiency is proportional to L in the optical wavelength converter devices of the Cerenkov radiation-type, but is proportional to the square of L in the optical wavelength converter devices of the waveguide-waveguide-type.
According to an optical wavelength converter device of the Cerenkov radiation-type, a beam of a wavelength-converted wave which is emitted from the device has a ring or cresent cross-sectional shape, and a special focusing optical system is required to focus the wavelength-converted wave into a small spot. According to an optical wavelength converter device of the waveguide-waveguide-type, however, if a wavelength-converted wave is guided in a single mode, then an emitted beam thereof substantially becomes a Gaussian beam. Therefore, the wavelength-converted wave can simply be converted down to the diffraction limit.
Conventional optical wavelength converter devices of the waveguide-waveguide-type which have heretofore been proposed have waveguides made of an inorganic nonlinear optical material. Consequently, the diameter of the core or the thickness of the optical waveguide, and the wavelength of a fundamental wave to be applied should be controlled highly accurately in order to achieve desired phase matching. In addition, the temperature of the device should be set to 100.degree. C. or higher and the device should be kept at that temperature within an error range of about .+-.0.1.degree. C. in order to achieve phase matching after the wavelength-converted wave has been guided in a single mode. The optical wavelength converter devices of the waveguide-waveguide-type have not yet been practical because of the various conditions or factors which need to be controlled strictly, as described above.